Flexible display device with wire having reinforced portion and manufacturing method for the same

ABSTRACT

There is provided a flexible display having a plurality of innovations configured to allow bending of a portion or portions to reduce apparent border size and/or utilize the side surface of an assembled flexible display.

BACKGROUND

1. Technical Field

This relates generally to electronic devices, and more particularly, toelectronic devices with a display.

2. Description of the Related Art

Electronic devices often include displays. For example, cellulartelephones and portable computers include displays for presentinginformation to a user. Components for the electronic device, includingbut not limited to a display, may be mounted in the housing made ofplastic or metal.

An assembled display may include a display panel and a number ofcomponents for providing a variety of functionalities. For instance, oneor more display driving circuits for controlling the display panel maybe included in a display assembly. Examples of the driving circuitsinclude gate drivers, emission (source) drivers, power (VDD) routing,electrostatic discharge (ESD) circuits, multiplex (mux) circuits, datasignal lines, cathode contacts, and other functional elements. There maybe a number of peripheral circuits included in the display assembly forproviding various kinds of extra functions, such as touch sense orfingerprint identification functionalities.

Some of the components may be disposed on the display panel itself,often in the areas peripheral to the display area, which is referred inthe present disclosure as the non-display area and/or the inactive area.When such components are provided in the display panel, they populate asignificant portion of the display panel. Large inactive area tends tomake the display panel bulky, making it difficult to incorporate it intothe housing of electronic devices. Large inactive area may also requirea significant portion of the display panel to be covered by overly largemasking (e.g., bezel, borders, covering material), leading tounappealing device aesthetics.

Size and weight are of the critical importance in designing modernelectronic devices. Also, a high ratio of the active area size comparedto that of inactive area, which is sometimes referred to as the screento bezel ratio, is one of the most desired feature. There is a limit asto how much reduction in the size of the inactive area for higherscreen-to-bezel ratio can be realized from mere use of a separateflexible printed circuit (FPC) for connecting components to the displaypanel. Space requirement for reliably attaching signal cables and to fanout wires along the edges of the display panel still needs to bedisposed in the inactive area of the display panel.

It will be highly desirable to bend the base substrate where the activewith the pixels and the inactive area are formed thereon. This wouldtruly minimize the inactive area of the display panel that needs to behidden under the masking or the device housing. Not only does thebending of the base substrate will minimize the inactive area size needto be hidden from view, but it will also open possibility to various newdisplay device designs.

However, there are various new challenges that need to be solved inproviding such flexible displays. The components formed directly on thebase substrate along with the display pixels tend to have tremendouslysmall dimension with unforgiving margin of errors. Further, thesecomponents need to be formed on extremely thin sheet to provideflexibility, making those components extremely fragile to variousmechanical and environmental stresses instigated during the manufactureand/or in the use of the displays.

Further complication arises from the fact that the components fabricateddirectly on the base substrate with the display pixels are often closelylinked to the operation of those pixels. If care is not taken, themechanical stresses from bending of the flexible display can negativelyaffect the reliability or even result in complete component failure.Even a micro-scale defect in the component thereof can have devastatingeffects on the performance and/or reliability of the display pixelsamounting to scrap the entire display panel without an option to repair.

For instance, a few micrometer scale cracks in the electric wires cancause various abnormal display issues and may even pixels in severalrows or sections of the display panel not to be activated at all. Assuch, various special parameters must be taken in consideration whendesigning electrical wiring schemes to be fabricated on the flexiblebase substrate along with the display pixels. Simply increasing thebending radius may make it difficult to garner any significant benefitsin flexing the base substrate of the display panel. It would thereforebe desirable to provide a flexible display that can operate reliablyeven under the bending stresses from extreme bending radius.

BRIEF SUMMARY

An aspect of the present disclosure is related to a flexible display,which includes configurations for wire traces to withstand bendingstress for reliable operation of the flexible display.

In an embodiment, a flexible display includes a substantially flatportion having a thin-film transistor (TFT) disposed on a base layer anda buffer layer interposed between the base layer and a semiconductorlayer of the TFT; a bend portion having a protruded area and a recessedarea, wherein the protruded area includes the buffer layer disposed onthe base layer and the recessed areas has the base layer without thebuffer layer disposed thereon; and a conductive line trace having aportion laid in the substantially flat portion of the flexible displayand another portion laid in the bend portion of the flexible display.

In an aspect, the present disclosure relates to a flexible organic lightemitting diode (OLED) display. The flexible OLED display includes asubstantially flat portion having a base layer, a buffer layer on thebase layer and a thin-film transistor (TFT) disposed on the bufferlayer; a bend portion having the base layer and the buffer layer on thebase layer, the bend portion being curved away from a tangent plane ofthe substantially flat portion by a bend angle; and a conductive linetrace extending from the substantially flat portion to the bend portionof the flexible display, the conductive line trace in the bend portionhaving a bend stress relief trace design in which the conductive linetrace splits into a plurality of sub-traces that merge back at one ormore intervals, wherein the buffer layer in the bend portion correspondsto the bend stress relief trace design of the conductive line trace inthe bend portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate schematic view of an exemplary flexible displaydevice according to embodiments of the present disclosure.

FIGS. 2A-2B illustrate schematic view of an exemplary flexible displaydevice according to embodiments of the present disclosure.

FIG. 3 illustrates schematic plane view and correspondingcross-sectional view of bending pattern, which may be employed inembodiments of the present disclosure.

FIGS. 4A-4B illustrate schematic view of an exemplary multi-layeredconductive lines usable in a flexible display device according toembodiments of the present disclosure.

FIGS. 5A-5B illustrate schematic view of an exemplary configuration ofmulti-layered conductive lines and insulation layers according toembodiments of the present disclosure.

FIGS. 6A-6B illustrate schematic view of an exemplary configuration ofrecessed channel and crack deflection metal/insulation trace accordingto embodiments of the present disclosure.

FIG. 7 is a schematic view of single line wire trace design usable forflexible displays according to an embodiment of the present disclosure.

FIGS. 8A-8D illustrate schematic view of an exemplary wire traces havinga plurality of sub-traces that split and merge at a certain intervalaccording to embodiments of the present disclosure.

FIGS. 9A-9B illustrate schematic view of an exemplary wire tracescrossing recessed area of the flexible display according to embodimentsof the present disclosure.

DETAILED DESCRIPTION

FIGS. 1A-1C illustrate exemplary flexible display 100 which may beincorporated in electronic devices. Referring to FIG. 1A, the flexibledisplay 100 includes at least one active area (i.e., display area), inwhich an array of display pixels are formed therein. One or moreinactive areas may be provided at the periphery of the active area. Thatis, the inactive area may be adjacent to one or more sides of the activearea. In FIG. 1A, the inactive area surrounds a rectangular shape activearea. However, it should be appreciated that the shapes of the activearea and the arrangement of the inactive area adjacent to the activearea are not particularly limited as the exemplary flexible display 100illustrated in FIG. 1A. The active area and the inactive area may be inany shape suitable to the design of the electronic device employing theflexible display 100. Non-limiting examples of the active area shapes inthe flexible display 100 include a pentagonal shape, a hexagonal shape,a circular shape, an oval shape, and more.

Parts of the flexible display 100 may be defined by a central portionand a bend portion. In reference to a bend line BL, the part of theflexible display 100 that remains substantially flat is referred to asthe central portion or the substantially flat portion, and the otherpart of the flexible display 100 at the other side of the bend line BLis referred to as the bend portion. Within a bend portion, the part thathas a curvature in an inclination angle or in a declination angle fromthe substantially flat portion may be specified as the bend allowancesection of the bend portion.

Multiple portions of the flexible display 100 can be bent. Accordingly,one or more edges of the flexible display 100 can be bent away from theplane of the central portion along the several bend lines BL. The bendline BL may extend horizontally (e.g., X-axis shown in FIG. 1A),vertically (e.g., Y-axis shown in FIG. 1A) or even diagonally in theflexible display 100. While the bend line BL is depicted as beinglocated near the edges of the flexible display 100, but it should beappreciated that the location the bend lines BL can be positioned in themid-area of the central portion. Any one or more corners of the flexibledisplay 100 may be bent as well. The flexible display 100 can be bent inany combination of horizontal, vertical and/or diagonal directions basedon the desired design of the flexible display 100. The bend line BL maybe placed across the central portion of the flexible display 100 toprovide a foldable display or a double-sided display having displaypixels on both outer sides of a folded display.

Each pixel in the active area may be associated with a pixel circuit,which includes at least one switching thin-film transistor (TFT) and atleast one driving TFT. Each pixel circuit may be electrically connectedto a gate line and a data line, and communicates with the drivingcircuits, such as a gate driver and a data driver, positioned in theinactive area of the flexible display 100 to operate the associatedpixel.

The flexible display 100 may include various additional components forgenerating a variety of signals or otherwise operating the pixels in theactive area. For example, an inverter circuit, a multiplexer, an electrostatic discharge (ESD) circuit and the like may be placed in theinactive area of the flexible display 100. The flexible display 100 mayalso include components associated with functionalities other than foroperating the pixels of the flexible display 100. For instance, theflexible display 100 may include components for providing a touchsensing functionality, a user authentication functionality (e.g., fingerprint scan), a multi-level pressure sensing functionality, a tactilefeedback functionality and/or various other functionalities for theelectronic device employing the flexible display 100.

Some of the components may be mounted on a separate printed circuit andcoupled to a connection interface (Pads/Bumps) disposed in the inactivearea using a printed circuit film such as flexible printed circuit board(FPCB), chip-on-film (COF), tape-carrier-package (TCP) or any othersuitable technologies. The inactive area with the connection interfacecan be bent away from the central portion so that the printed circuitfilm such as the COF, FPCB and the like are positioned at the rear sideof the flexible display 100.

Some of the components may be positioned in the inactive area of theflexible display 100. For example, one or more driving circuits may beimplemented with TFTs fabricated in the inactive area as depicted inFIG. 1A. Such gate drivers may be referred to as a gate-in-panel (GIP).

For convenience, the pixel circuit and the driving circuits in theembodiments of flexible display 100 will be described as beingimplemented with TFTs using low-temperature poly-silicon (LTPS)semiconductor layer as its active layer. Accordingly, in someembodiments, the pixel circuit and the driving circuits (e.g., gatedriver) are implemented with NMOS LTPS TFTs having a co-planarstructure. In some embodiments, the driving circuits in the inactivearea of the flexible display 100 may be implemented with a combinationof N-Type LTPS TFTs and P-Type LTPS TFTs.

It should be noted that some embodiments of the flexible display 100 canemploy multiple types of TFTs to implement the driving circuits in theinactive area and/or the pixel circuits in the active. The type of TFTsmay be selected according to the operating conditions and/orrequirements of the TFTs in the corresponding circuit.

A low-temperature-poly-silicon (LTPS) TFT generally exhibits excellentcarrier mobility even at a small profile, making it suitable forimplementing condensed driving circuits. The excellent carrier mobilityof the LTPS TFT makes it an ideal for components requiring a fast speedoperation. Despite the aforementioned advantages, initial thresholdvoltages may vary among the LTPS TFTs due to the grain boundary of thepoly-crystalized silicon semiconductor layer.

TFTs using other kinds of semiconductor layers may be different from theLTPS TFT in many respects. For instance, a TFT employing an oxidematerial based semiconductor layer such as an indium-gallium-zinc-oxide(IGZO) semiconductor layer, which is referred hereinafter as “the oxideTFT,” is generally more advantageous than the LTPS TFT in terms ofreducing the leakage current during operation of the display. Inaddition, the oxide TFT generally exhibits a higher voltage-holdingratio (VHR) than that of the LTPS TFT, albeit providing a lower mobilitythan the LTPS TFT. The higher VHR of the oxide TFT is more suitable fordriving the pixels at a reduced frame rate when a high frame ratedriving of the pixels is not needed. Accordingly, the pixel circuits maybe implemented with the oxide TFTs in some embodiments of the flexibledisplay 100 provided with a feature in which all or selective pixels aredriven at a reduced frame rate (e.g., 30 frames per second) from anormal frame rate under a specific condition. Considering the benefitsof the oxide TFTs mentioned above, a power efficient and energy savingflexible display 100 can be provided with the oxide TFT.

The oxide TFT does not suffer from the initial threshold voltagevariation issue of the LTPS TFT, which can be a great advantage for alarge sized flexible display 100. On the other hand, the LTPS TFT mayfair better than the oxide TFT in terms of the positive bias temperaturestress (PBTS) and the negative bias temperature stress (NBTS), which maycause unwanted threshold voltage shift during the use of the flexibledisplay 100.

Considering the pros and cons of LTPS TFT and oxide TFT, someembodiments of the flexible display 100 disclosed herein may employ acombination of the LTPS TFT and the oxide TFT. In particular, someembodiments of the flexible display 100 can employ LTPS TFTs toimplement the driving circuits (e.g., GIP) in the inactive area andemploy oxide TFTs to implement the pixel circuits in the active area.Due to the excellent carrier mobility of the LTPS TFT, driving circuitsimplemented with LTPS TFTs may operate at a faster speed than thedriving circuits implemented with the oxide TFTs. In addition, the LTPSTFT enables to provide more condensed driving circuits, which in turn,leading to a smaller sized inactive area in the flexible display 100.With excellent voltage holding ratio of the oxide TFTs used in the pixelcircuits, the leakage current from the pixels can be reduced to lowerthe power consumption of the flexible display 100. This also enables torefresh pixels in a selective portion of the active area or to drivepixels at a reduced frame rate under a predetermined condition (e.g.,when displaying still images) while minimizing possible display defectsassociated with the leakage current. In some embodiments, the drivingcircuits in the inactive area of the flexible display 100 may beimplemented with a combination of N-Type LTPS TFTs and P-Type LTPS TFTswhile the pixel circuits are implemented with oxide TFTs.

In some embodiments, the display driving circuits and/or peripheralcircuits in the inactive area may be implemented by using both LTPS TFTsand oxide TFTs. Likewise, in some embodiments, the pixel circuits in theactive area can be implemented by using both the LTPS TFTs and the oxideTFTs. For instance, the LTPS TFT can be used for TFTs of a drivingcircuit and/or in a pixel circuit, which are subjected to extendedperiod of bias stress (e.g., PBTS, NBTS). In addition, the TFTs in adriving circuit and/or a pixel circuit, which are connected to a storagecapacitor, can be formed of the oxide TFT to stably maintain thecapacitance in the storage capacitor.

FIG. 1B is a simplified cross-sectional view of an exemplary flexibledisplay. As illustrated in FIG. 1B, the central portion of the flexibledisplay 100 may be substantially flat, and one or more bend portions maybe bent away from the tangent vector of the curvature at a certain bendangle and a bend radius around the bending axis. The size of each bendportion that is bent away from the central portion needs not be thesame. That is, the length of the base layer 106 from the bend line BL tothe outer edge of the base layer 106 at each bend portion can bedifferent. Also, the bend angle around the bend axis and the bend radiusfrom the bend axis can vary between the bend portions.

In the example shown in FIG. 1B, one of the bend portion (right side)has the bend angle θ of 90°, and the bend portion includes asubstantially flat section. The bend portion can be bent at a largerbend angle θ, such that at least some part of the bend portion comesunderneath the plane of the central portion of the flexible display 100as one of the bend portion (left side). Also, a bend portion can be bentat a bend angle θ that is less than 90°.

In some embodiments, the radius of curvatures (i.e., bend radius) forthe bend portions in the flexible display 100 may be between about 0.1mm to about 10 mm, more preferably between about 0.1 mm to about 5 mm,more preferably between about 0.1 mm to about 1 mm, more preferablybetween about 0.1 mm to about 0.5 mm. The lowest bend radius of the bendportion of the flexible display 100 may be less than 0.5 mm.

As shown in FIG. 1B, an organic light-emitting diode (OLED) elementlayer 102 is disposed on the base layer 106, and an encapsulation layer104 is disposed on the organic light-emitting diode (OLED) element layer102. The flexible display 100 also includes a support member 116, whichmay be referred to as a “mandrel.” The support member 116 has a planarbody portion and a rounded end portion. The base layer 106 and thesupport member 116 are arranged so that that the rounded end portion ofthe support member 116 is positioned in the bend portion of the baselayer 106. In other words, the rounded end portion of the support member116 facilitates to maintain the bend radius at the bend portion of thebase layer 106. As shown in FIG. 1B, the bend portion of the base layer106 may have a size that is sufficient to have a part of the base layer106 come under the support member 116. In this way, the circuits mountedon the chip on flex (COF) cable and/or the printed circuit board can beplaced at the backside of the flexible display 100.

The flexible display 100 includes one or more support layers 108 forproviding rigidity at the selective portion of the flexible display 100.Accordingly, the support layer 108 should be more rigid than the baselayer 106. The support layer 108 is attached on the inner surface of thebase layer, which is the opposite side of the surface having the OLEDelement layer 102 disposed thereon. Increase in the rigidity atselective parts of the flexible display 100 may help in ensuring theaccurate configuration and placement of the components of the flexibledisplay 100.

The base layer 106 and the support layer 108 may each be made of a thinplastic film formed from polyimide, polyethylene naphthalate (PEN),polyethylene terephthalate (PET), other suitable polymers, a combinationof these polymers, etc. Other suitable materials that may be used toform the base layer 106 and the support layer 108 include, metal foilcovered with a dielectric material, a multi-layer polymer stack, a thinglass film bonded to a thin polymer, a polymer composite film comprisinga polymer material combined with nanoparticles or micro-particlesdispersed therein, etc.

Excessive thickness of the base layer 106 makes it harder to be bent atextremely small bending radius desired by the flexible display 100.Excessive thickness of the base layer 106 may also increase themechanical stress to the components disposed thereon on the base layer106. As such, the thickness of the base layer 106 may depend on the bendradius at the bend portion of the base layer 106. On the other hand, thebase layer 106 with a thickness below a certain level may not be strongenough to reliably support various components disposed thereon.

Accordingly, the base layer 106 may have a thickness in a range of aboutfrom 5 μm to about 50 μm, more preferably in a range of about 5 μm toabout 30 μm, and more preferably in a range of about 5 μm to about 16μm. The support layer 108 may have a thickness from about 100 μm toabout 125 μm, from about 50 μm to about 150 μm, from about 75 μm to 200μm, less than 150 μm, or more than 100 μm. In one suitable exemplaryconfiguration, the base layer 106 is formed from a layer of polyimidewith a thickness of about 10 μm and the support layer 108 is formed frompolyethylene terephthalate (PET) with a thickness of about 100 μm toabout 125 μm.

During manufacturing or in normal use of the flexible display 100, partof the flexible display 100 may be exposed to external light. Some ofthe components or materials used in fabricating the components disposedon the base layer 106 may be sensitive to the external light, andundesirable state changes may occur upon exposure to the light ofcertain wavelength and/or exposure to the light amount of period. Someparts of the flexible display 100 may be more heavily exposed to theexternal light than others, and this can lead to a displaynon-uniformity (e.g., mura, shadow defects, etc.). To minimize suchproblems, the base layer 106 and/or the support layer 108 may includeone or more materials capable of reducing the amount of external lightpassing through.

In way of an example, the light blocking material, for instance chloridemodified carbon black, may be mixed in the constituent material of thebase layer 106 (e.g., polyimide) so that the base layer 106 can providesa light blocking functionality. As such, the base layer 106 may formedof polyimide with a shade. In addition to reducing undesired effectscaused by the light coming in from the rear side of the flexible display100, such a shaded base layer 106 can improve the visibility of thedisplayed image content by reducing the reflection of the external lightcoming in from the front side of the flexible display 100. Instead ofthe base layer 106, the support layer 108 may include a light blockingmaterial to reduce the amount of light coming in from the rear side(i.e., the support layer 108 attached side) of the flexible display 100.The constituent material of the support layer 108 may be mixed with oneor more light blocking materials in the similar fashion as describedabove with the base layer 106.

In some embodiments, both the base layer 106 and the support layer 108can include one or more light blocking materials. Here, the lightblocking materials used in the base layer 106 and the support layer 108need not be the same.

While making the base layer 106 and the support layer 108 to block theunwanted external light may improve display uniformity and reducereflection as described above, it can create a number of difficultiesduring manufacturing of the flexible display 100. When the base layer106 and the support layer 108 are non-transmittable to an excessiverange of wavelengths of light, recognizing the alignment marks on theselayers during their alignment process may not be easy. In particular,accurate positioning of the components on the base layer 106 or thealignment during bending of the flexible display 100 can becomedifficult, as the positioning of the layers may need to be determined bycomparing the outer edges of the overlapping portions of the layer(s).Further, checking for unwanted debris or other foreign materials in theflexible display 100 can be problematic if the base layer 106 and/or thesupport layer 108 blocks the excessive range(s) of light spectrum (i.e.,wavelengths in the visible, the ultraviolet and the infrared spectrum).

Accordingly, in some embodiments, the light blocking material, which maybe included in the base layer 106 and/or the support layer 108 isconfigured to pass the light of certain polarization and/or the lightwithin specific wavelength ranges usable in one or more manufacturingand/or testing processes of the flexible display 100. In way of anexample, the support layer 108 may pass the light used in the qualitycheck, alignment processes (e.g., UV, IR spectrum light) during themanufacturing the flexible display 100 but filter the light in thevisible light wavelength range. The limited range of wavelengths canhelp reduce the display non-uniformity problem, which may be caused bythe shadows generated by the printed circuit film attached to base layer106, especially if the base layer 106 includes the light blockingmaterial as described above.

It should be noted that the base layer 106 and the support layer 108 canwork together in blocking and passing specific types of light. Forinstance, the support layer 108 can change the polarization of light,such that the light will not be passable through the base layer 106.This way, certain type of light can penetrate through the support layer108 for various purposes during manufacturing of the flexible display100, but unable to penetrate through the base layer 106 to causeundesired effects to the components disposed on the opposite side of thebase layer 106.

The flexible display 100 may also include a polarizer layer 110 forcontrolling the display characteristics (e.g., external lightreflection, color accuracy, luminance, etc.) of the flexible display100. A cover layer 114 may be used to protect the flexible display 100.Electrodes for sensing touch input from a user may be formed on aninterior surface of a cover layer 114 and/or at least one surface of thepolarizer layer 110.

The flexible display 100 may further include a separate layer thatincludes the touch sensor electrodes and/or other components associatedwith sensing of touch input (referred hereinafter as touch-sensor layer112). The touch sensor electrodes (e.g., touch driving/sensingelectrodes) may be formed from transparent conductive material such asindium tin oxide, carbon based materials like graphene or carbonnanotube, a conductive polymer, a hybrid material made of mixture ofvarious conductive and non-conductive materials. Also, metal mesh (e.g.,aluminum mesh, silver mesh, etc.) can also be used as the touch sensorelectrodes.

The touch sensor layer 112 may include a layer formed of one or moredeformable materials. One or more electrodes may be interfaced with orpositioned near the deformable material, and loaded with one or moresignals to facilitate measuring electrical changes on one or more of theelectrodes upon deformation of the deformable material. The measurementmay be analyzed to assess the amount of pressure at a plurality ofdiscrete levels and/or ranges of levels on the flexible display 100. Insome embodiments, the touch sensor electrodes can be utilized inidentifying the location of the user inputs as well as assessing thepressure of the user input. Identifying the location of touch input andmeasuring of the pressure of the touch input on the flexible display 100may be achieved by utilizing at least one common signal. Also, measuringthe amount of pressure may utilize at least one electrode other than thetouch sensor electrodes to measure at least one other signal, which maybe obtained simultaneously with the touch signal from the touch sensorelectrodes or obtained at a different timing.

In some embodiments, the deformable material may be electro-activematerials, which the amplitude and/or the frequency of the deformationis controlled by an electrical signal and/or electrical field. Theexamples of such deformable materials include piezo ceramic,electro-active-polymer (EAP) and the like. Accordingly, the touch sensorelectrodes and/or separately provided electrode can activate thedeformable material to bend the flexible display 100 to desireddirections. In addition, such electro-active materials can be activatedto vibrate at desired frequencies, thereby providing tactile and/ortexture feedback on the flexible display 100. It should be appreciatedthat the flexible display 100 may employ a plurality of electro-activematerial layers so that bending and vibration of the flexible display100 can be provided simultaneously or at a different timing. Such acombination can be used in providing sound directly from the flexibledisplay 100 without a separate speaker.

As mentioned above, bending the inactive area allows to minimize or toeliminate the inactive area seen from the front side of the assembledflexible display 100. If the bend portion does not cover the entireinactive area on the side of the flexible display 100, the inactive areathat remains visible from the front side can be covered with a bezel.The bezel may be formed, for example, from a stand-alone bezel structurethat is mounted to a housing of the electronic device, from a portion ofhousing (e.g., a portion of the sidewalls of housing), or using othersuitable structures. The inactive area remaining visible from the frontside may also be hidden under an opaque masking layer, such as black ink(e.g., a polymer filled with carbon black) or a layer of opaque metal.Such an opaque masking layer may be coated on a portion of layersincluded in the flexible display 100, such as the touch sensor layer112, the polarizer layer 110, the cover layer 114, and other suitablelayers.

While the central portion of the flexible display 100 has a flat surfacein FIG. 1B, some embodiments may not have such a flat central portion.The central portion of the flexible display 100 can be curved-in orcurved-out as depicted in FIG. 1C, providing flexible display 100 with aconcave or a convex central portion. Even in the embodiments with aconvex or concave curved central portion, one or more bend portions ofthe flexible display 100 can be bent inwardly or outwardly along thebend line at a bend angle around a bend axis.

Referring back to FIG. 1A, the bend portion of the flexible display 100may include an active area capable of displaying image from the bendportion, which is referred herein after as the secondary active area.That is, the bend line BL can be positioned in the active area so thatat least some display pixels of the active area is included in the bendportion of the flexible display 100. In this case, the matrix of pixelsin the secondary active area of the bend portion may be continuouslyextended from the matrix of the pixels in the active area of the centralportion as depicted in FIG. 2A. Alternatively, the secondary active areawithin the bend portion and the active area within the central portionof the flexible display 100 may be separated apart from each other bythe outer bend radius as depicted in FIG. 2B.

The secondary active area in the bend portion may serve as a secondarydisplay area in the flexible display 100. The size of the secondaryactive area is not particularly limited. The size of the secondaryactive area may depend on its functionality within the electronicdevice. For instance, the secondary active area may be used to provideimages and/or texts such a graphical user interface, buttons, textmessages, and the like. In some cases, the secondary active area may beused to provide light of various colors for various purposes (e.g.,status indication light), and thus, the size of the secondary activearea need not be as large as the active area in the central portion ofthe flexible display 100.

The pixels in the secondary active area and the pixels in the centralactive area may be addressed by the driving circuits (e.g., gate driver,data driver, etc.) as if they are in a single matrix. In this case, thepixels of the central active area and the pixels of the secondary activearea may be operated by the same set of signal lines (e.g., gate lines,emission lines, etc.). In way of example, the Nth row pixels of thecentral active area and the Nth row pixels of the secondary active areamay be configured to receive the gate signal from the same gate driver.As shown in FIG. 2B, the part of the gate line crossing over the bendallowance section (i.e., bend allowance region) or a bridge forconnecting the gate lines of the two active areas may have a split tracedesign made of at least two sub-traces, which will be described infurther detail below.

In some embodiments, the pixels in the secondary active area may bedriven discretely from the pixels in the central active area. That is,the pixels of the secondary active area may be recognized by the displaydriving circuits as being an independent matrix of pixels separate fromthe matrix of pixels in the central active area. In such cases, thepixels of the central active area and the pixels of the secondary activearea may utilize different set of signal lines from each other. Further,the secondary active area may be employ one or more display drivingcircuits discrete from the ones employed by the central active area.

Components of the flexible display 100 may make it difficult to bend theflexible display 100 along the bend line BL. Some of the elements, suchas the support layer 108, the touch sensor layer 112, the polarizerlayer 110 and the like, may add too much rigidity to the flexibledisplay 100. Also, the thickness of such elements can make otherelements of the flexible display 100 subjected to greater bendingstresses as well.

To facilitate easier bending and to enhance the reliability of theflexible display 100, the configuration of components in one portion ofthe flexible display 100 may differ from other portion of the flexibledisplay 100. In other words, some of the components may not be disposedon one or more portions of the flexible display 100, or may havedifferent thicknesses at different portions of the flexible display 100.Accordingly, the bend portion may free of the support layer 108, thepolarizer layer 110, the touch sensor layer 114, a color filter layerand/or other components that may hinder bending of the flexible display100. Such components may not be needed in the bend portion if the bendportion is to be hidden from the view or otherwise inaccessible to theusers of the flexible display 100.

Even if the secondary active area is in the bend portion for providinginformation to users, the secondary active area may not require some ofthese components depending on the usage and/or the type of informationprovided by the secondary active area. For example, the polarizer layer110 and/or color filter layer may not be needed in the bend portion whenthe secondary active area is used for simply emitting colored light,displaying texts or simple graphical user interfaces in a contrast colorcombination (e.g., black colored texts or icons in white background).Also, the bend portion of the flexible display 100 may be free of thetouch sensor layer 114 if such functionality is not needed in the bendportion. If desired, the bend portion may be provided with a touchsensor layer 112 and/or the layer of electro-active material even thoughthe secondary active area for displaying information is not provided inthe bend portion.

Also, some of the components typically placed in the central portion maybe separated from the respective components in the bend portion toreduce unwanted cracks or damages due to the bend stress. To this end,some of the elements in the central portion may not be formed in atleast some part of the bend portion. The separation between thecomponents in the central portion and the bend portion may be created byselectively removing the elements at the bend allowance section of theflexible display 100 such that the bend allowance section is free of therespective elements.

In some embodiments, at least part of the bend allowance section in theflexible display 100 can be free of the support layer 108, therebyseparating the support layer 108A in the central portion and the supportlayer 108B in the bend portion underside of the base layer 106 asillustrated in FIG. 2B. Components and/or layers disposed on the baselayer 106, for example the polarizer layer 110 and the touch sensorlayer 114, in the central portion and the bend portion may also beseparated by a part or by the entire length of the bend allowancesection of the flexible display 100. Some components in the centralportion may be electrically connected to the components in the bendportion via one or more conductive line trace 120 laid across the bendallowance section of the flexible display 100.

The removal of the elements may be done by cutting, wet etching, dryetching, scribing and breaking, or other suitable material removalmethods. Rather than cutting or otherwise removing an element, separatepieces of the element may be formed at the central portion and the bendportion to leave the bend allowance section free of such element.

In order to reduce the bend stress, some elements may be provided with abend pattern along the bend lines or otherwise within the bend allowancesection rather than being entirely removed from the bend portion. FIG. 3illustrates a plane view and a cross-sectional view of exemplary bendpatterns 300. It should be noted that the flexible display 100 mayutilize more than one types of bend patterns. If desired, the depth ofthe patterns may not be deep enough to penetrate through the componententirely but penetrate only partially through the respective layer. Itshould be noted that the order of the bend patterns illustrated in theplane view of the flexible display 100 do not necessarily match with theorder of bend patterns illustrated in the cross sectional view of theflexible display 100 in FIG. 3. The bend patterns 300 described abovemay be used in the support layer 108, the polarizer layer 110, the touchsensor layer 114 and various other elements of the flexible display 100.In addition, a buffer layer positioned between the base layer 106 andthe TFT as well as a passivation layer covering a conductive line tracemay be also be provided with the bend pattern for reducing bend stress.It should be appreciated that a number of bend patterns and the types ofthe bend patterns 300 utilized by the components is not limited.Accordingly, an element may utilize several types of bend patterns 300.

Several conductive lines are included in the flexible display 100 forelectrical interconnections between various components therein. Thecircuits fabricated in the active area and inactive area may transmitvarious signals via one or more conductive lines to provide a number offunctionalities in the flexible display 100. As briefly discussed, someconductive lines may be used to provide interconnections between thecircuits and/or other components in the central portion and the bendportion of the flexible display 100.

In the present disclosure, the conductive lines may include source/drainelectrodes of the TFTs as well as the gate lines/data lines used intransmitting signals from some of the display driving circuits (e.g.,gate driver, data driver) in the inactive area to the pixel circuits inthe active area. Likewise, some conductive lines, such as the touchsensor electrodes, pressure sensor electrodes and/or fingerprint sensorelectrodes may provide signals for sensing touch input or recognizingfingerprints on the flexible display 100. The conductive lines can alsoprovide interconnections between the pixels of the active area in thecentral portion and the pixels of the secondary active area in the bendportion of the flexible display 100. Aforementioned uses of conductivelines are merely illustrative. As used herein, the conductive linesbroadly refers to a conductive path for transmitting any type ofelectrical signals, power and/or voltages from one point to anotherpoint in the flexible display 100.

Some of the conductive lines may be extended from the substantially flatportion of the flexible display to the bend portion of the flexibledisplay 100. In such cases, some portions of the conductive lines may beconfigured differently from the other portions to withstand the bendingstress. In particular, the portion of the conductive lines laid over thebend allowance section of the flexible display 100 may include severalfeatures for reducing cracks and fractures of the conductive lines tomaintain proper interconnection.

Some of the conductive lines in the flexible display 100 may have amulti-layered structure, which may allow more stretching (orflexibility) with less chance of breakage. FIGS. 4A and 4B eachillustrates exemplary stack structure of the conductive line 120. InFIG. 4A, the conductive line 120 may have a multi-layered structure inwhich the primary conductive layer 122 is sandwiched between thesecondary conductive layers 124.

The primary conductive layer 122 may be formed of material with a lowerelectrical resistance than that of the secondary conductive layer 144.Non-limiting examples of the materials for the primary conductive layer122 includes copper, aluminum, transparent conductive oxide, or otherflexible conductors. The secondary conductive layer 124 should be formedof conductive material that can exhibit sufficiently low ohmic contactresistance when formed in a stack over the primary conductive layer 122.

Examples of such combination include an aluminum layer sandwichedbetween titanium layers (Ti/Al/Ti), an aluminum layer sandwiched betweenupper and lower molybdenum layers (Mo/Al/Mo), a copper layer sandwichedbetween titanium layers (Ti/Co/Ti) and a copper layer sandwiched betweenupper and lower molybdenum layers (Mo/Co/Mo). However, the low ohmiccontact resistance of the conductive layer stack is not the only factorfor choosing the materials for the conductive line 120 used in theflexible display 100.

With extreme bend radius requirement at the bend allowance section ofthe flexible display 100, the materials for forming the conductive linetrace 120 should meet the minimum Young's modulus requirement whilemeeting the stringent electrical requirements of the flexible display100. Accordingly, both the primary conductive layer 122 and thesecondary conductive layer 124 should be formed of materials exhibitinglow brittleness (E). In this regard, Al has a modulus of about 71 GPa,Ti has a modulus of 116 GPa, and Mo has a modulus of 329 GPa. As such,the brittleness (E) of Al is about ¼ of that of Mo, and the brittleness(E) of Ti is about ⅓ of that of Mo. Accordingly, in some embodiments, atleast some of the conductive lines 120 of the flexible display 100 areformed of a stack including Al and TI. Unlike Mo, both Al and Tiexhibited no cracks at the bend radius of 0.5 mm.

Since the primary conductive layer 122 should have lower electricalresistance than the secondary conductive layer 124, the conductive line120 may be formed in a stack of Ti/Al/Ti. In particular, at least someof the conductive lines 120 disposed in the bend allowance section maybe formed in a stack of Ti/Al/Ti.

In some embodiments, the flexible display 100 may be employed in awearable electronic device. In such cases, the flexible display 100 maybe operating under highly humid environment. In some cases, sweat of theuser may penetrate in the device housing, and corrode some of theconductive line trace 120. Dissimilar metals and alloys have differentelectrode potentials, and when two or more come into contact in anelectrolyte, one metal acts as anode and the other as cathode. Theelectro-potential difference between the dissimilar metals is thedriving force for an accelerated attack on the anode member of thegalvanic couple, which is the primary conductive layer 202 in theTi/Al/Ti stack. The anode metal dissolves into the electrolyte, anddeposit collects on the cathodic metal. Due to Al wire corrosion,electrical characteristics of the conductive line trace 120 may bedeteriorated (withstand voltage may be lowered, etc.), and wire breakagemay even occur.

Typically, galvanic corrosion is initiated by the electrolyte that is incontact at the cross-sectional side of a stack structured wire.Accordingly, at least some conductive lines 120 of an embodiments mayhave a structure in which the primary conductive layer 122 is surroundedby the secondary conductive layer 124 such that even the two side endsof the primary conductive layer 122 are covered by the secondaryconductive layer 124 as shown in FIG. 4B. This can minimize the loss ofprimary conductive layer 202 by galvanic corrosion, and further reduceprobability of severance of electrical conductivity.

Such a multi-layered conductive lines 120 can be created by firstdepositing the material for the primary conductive layer 122 (e.g., Al)over the secondary conductive layer 124 (e.g., Ti). Here, the secondaryconductive layer 124 underneath the primary conductive layer 122 mayhave greater width. Etch resist material is formed over the stack ofthese two layers and etched (e.g., dry etch, wet etch, etc.) to form adesired a conductive line trace (e.g., diamond trace design). Afterstriping the etch resistance material, another layer of secondaryconductive layer 124 (i.e., Ti) is deposited over the patternedstructure (i.e., Ti/Al). Again, the secondary conductive layer 124 ontop of the primary conductive layer 122 may have greater width such thatthe primary conductive layer 122 is enclosed within the secondaryconductive layer 124. Another round of dry etching and striping of theetch resistance material is performed to form the stack of themulti-layered structure (i.e., Ti/Al/Ti) in a desired conductive linetrace design.

Various insulation layers, such as the a buffer layer 126, thepassivation layer 128, a gate insulation layer (GI layer) and aninterlayer dielectric layer (ILD layer) may be formed at the lowerand/or upper side of the conductive line trace 120. These insulationlayers may be formed of organic and/or inorganic materials or include asub-layer formed of inorganic materials, which are generally lessductile than the metals of the conductive lines 120.

Given the same amount of bending stress, cracks generally initiate fromthe insulation layers for the conductive line trace 120. Even if theconductive lines trace 120 has sufficient modulus to withstand thebending stress without a crack, the cracks initiated from the insulationlayer tend to grow and propagate into the conductive lines 120, creatingspots of poor electrical contacts that could render the flexible display100 unusable. Accordingly, various bending stress reduction techniquesare utilized in both the insulation layers and the conductive lines 120.

It should be noted that cracks primarily propagate through inorganicinsulation layers. Accordingly, propagation of cracks can be suppressedby selectively removing inorganic insulation layers from the areas proneto cracks. Accordingly, the inorganic insulation layers and/or stack ofinsulation layers including an inorganic insulation layer can beselectively etched away at certain part of the flexible display 100.

For example, the insulation layer under the conductive line trace 120can be etched away as depicted in FIG. 5A. The insulation layer underthe conductive line 120 may be the buffer layer 126, which may includeone or more layers of inorganic material layers.

The buffer layer 126 is disposed on the base layer 126, but under theTFT. Also, the buffer layer 126 is formed of one or more layers of aSiN_(x) layer and a SiO₂ layer. The buffer layer 126 formed on thesubstantially flat portion of the base layer 106 may be thicker than thebuffer layer 126 over the bend portion of the base layer 106. Tofacilitate easier bending of the flexible display 100, a part of thebuffer layer 126 may etched away in the bend portion of the flexibledisplay 100. For example, the buffer layer 126 in the substantially flatportion may include multiple stacks of a SiN_(x) layer and a SiO₂ layer,and the buffer layer 126 in the bend portion includes a single stack ofa SiN_(x) layer and a SiO₂ layer. As such, the buffer layer 126 in thesubstantially flat portion of the flexible display 100 has at least oneadditional sub-layer than the buffer layer in the bend portion of theflexible display 100.

Each layer of a SiN_(x) layer and a SiO₂ layer may have a thickness ofabout 1000 Å. As such the thickness of the buffer layer in the bendportion of the flexible display may range from about 100 Å to about 2000Å. In the substantially flat portion of the flexible display 100, theupper most inorganic layer, which is the inorganic layer immediatelybelow the semiconductor layer of the TFT, has a thickness of about 3000Å.

In the bend allowance section, the buffer layer 126 may be etched evenfurther to expose the base layer 106 while leaving the buffer layer 126intact under the conductive line trace 120. In other words, a recessedarea and a protruded area are provided in the bend portion of theflexible display 100. The protruded area includes the buffer layer 126provided on the base layer 106, whereas the recessed area has the baselayer 106 exposed without the buffer layer 126 disposed thereon. Theconductive line trace 120 is positioned on the protruded area, and thepassivation layer 128 is positioned over the conductive line trace 120on the protruded area. Although the passivation layer 128 may betemporarily deposited over the recessed area, the passivation layer 128can be removed from the recessed area by dry etch or wet etch process.As such, the recessed area can be substantially free of the passivationlayer 128. When etching the passivation layer 128 from the recessedarea, part of the base layer 106 can also be etched. Accordingly, thethickness of the base layer 106 at the recessed area can be lower thanthat of the base layer 106 elsewhere in the flexible display 100. Whenthe buffer layer 126 is etched away as shown in FIG. 5A, the crackpropagation within the multi-buffer 126, which may propagate into theconductive line trace 120 can be prevented.

As illustrated in FIG. 5B, in some embodiments, the recessed areaincludes the base layer 106 that is etched to a certain depth, and theconductive line trace 120 is deposited on the base layer 106 of therecessed area. In this setting, the portion of the conductive line trace120 is disposed within the base layer 106. The conductive line trace 120is also deposited on a part of the buffer layer 126 that provides theprotruded area. A passivation layer 128 can be deposited over theconductive line trace 120, and then the passivation layer 128 is etchedaway from the recessed area to expose the conductive line 120 in therecessed area. Accordingly, the passivation layer 128 remains on theconductive line trace 120 positioned on the protruded area. In thisconfiguration, the passivation layer 128 remaining on the buffer layer126 inhibits galvanic corrosion as it covers the side surface of themulti-layered conductive line trace 120. Accordingly, the conductiveline trace 120 in the recessed area needs not be covered by thepassivation layer 128. With the passivation layer 128 removed from thesurface of the conductive line trace 120, crack propagation from thepassivation layer 128 can be prevented. Since galvanic corrosion startsfrom the edge of the conductive line trace 120 on the buffer layer, thepassivation layer 128 covering the edge of the conductive lines 120 onthe buffer 126 may not be needed if the distance between the conductiveline trace 120 on the buffer layer 126 and the conductive line trace 120in the base layer 106 is sufficiently far.

Crack can also occur in the insulation layers during scribing and/orchamfering some part of the flexible display 100. The cracks generatedat the edge of the flexible display 100 during such manufacturingprocesses can propagate towards central portion of the flexible display100. In some cases, cracks at the edge of the side inactive areaspropagate towards the active area and damage GIPs in the inactive areas.Accordingly, in some embodiments, a recessed channel can be formed inthe inactive area by etching the insulation layers to a desired depth asshown in FIG. 6A. Etching of the insulation layers can be done near theboundary of central portion and the bend portion. More particularly,near the start of the bend allowance section. If desired, the recessedchannel can be formed near the end of the bend allowance section. Itshould be noted that the recessed channel needs not be exactly at thebend line where the bend allowance section begins, but it can bepositioned towards the central portion or inside the bend allowancesection. In some embodiments, the recessed channel can be formed in theside inactive area between the GIP and outer edge of the inactive area.This way, propagation of cracks towards the GIP can be suppressed by therecessed channel by the recessed channel.

In some embodiments, a metal and insulation layer pattern capable ofchanging the direction of crack can be formed between a circuitpositioned in the inactive area and the outer edge of the inactive area.For example, a diamond shaped metal trace and insulation layer coveringthe metal trace can be formed between the GIP and the outer edge of theflexible display 100 as depicted in FIG. 6B. In this configuration, thecracks propagating from the outer edge of the inactive area in thedirection towards the GIP would change its course due to the angle ofthe diamond metal/insulation trace formed before the GIP.

However, complete removal of inorganic insulation layers, such as SiNx,can affect the electrical characteristic of components in the flexibledisplay 100. For instance, undesired shift in the threshold voltage ofTFTs was observed when SiNx layers were removed from the buffer layer126. As such, in some embodiments, an additional metal layer is formedunder the semiconductor layer of the TFTs, and the metal layer waselectrically connected to the source electrode or gate electrode tomaintain reliable operability of the TFT.

A trace designs plays an important role in reducing the bending stressin both the conductive line trace 120 and the insulation layers. Forconvenience of explanation, the conductive line trace 120 and the traceof insulation layer (i.e., passivation layer 128) covering at least somepart of the conductive line trace 120 are collectively referred to asthe “wire trace” in the following description.

The trace design should be determined by considering the electricalrequirements of the conductive line trace 120 as well as the type ofsignals transmitted on the conductive line trace 120. Also, theproperties of the materials (e.g., Young's modulus) used in forming theconductive line trace 120 and the insulation layers can be considered indesigning the traces. It should be noted that various other factors suchas a thickness, a width, a length, a layout angle for a section as wellas for the entirety of the conductive line trace 120 and the insulationlayers may need to be considered to provide a trace design havingsufficient electrical and mechanical reliability for use in the flexibledisplay 100.

The wire trace design may be specifically tailored for the conductiveline trace 120 and the insulation layers based on their placement andorientation in reference to the bending direction (i.e., tangent vectorof the curve) of the flexible display 100. A wire trace will besubjected to more bending stress as the direction in which the wiretrace extends is more aligned to the tangent vector of the curvature. Inother words, a wire trace will withstand better against the bendingstress when the length of the wire trace aligned to the tangent vectorof the curvature is reduced.

In order to reduce the length of the wire trace portion being aligned tothe tangent vector of the curvature, wire traces of the flexible display100 may employ any one or more of a sign-wave, a square-wave, aserpentine, a saw-toothed and a slanted line trace designs illustratedin FIG. 7. In such configurations, the bending stress may be distributedto the trace portions oriented in an angle shifted away from the tangentvector of the curvature. The strain reducing trace designs illustratedin FIG. 7 are merely exemplary and should not be construed aslimitations to the trace designs that can be used in the embodiments ofthe flexible display 100.

Some conductive line trace 120 may adopt different strain reducing tracedesigns from other conductive line trace 120 of the flexible display100. In some embodiments, the conductive line trace 120 can have varyingdimensions to facilitate tight spacing between the conductive lines. Forinstance, a convex side of a first wire trace may be placed in a concaveside of a second wire trace next to the first wire trace.

Since the cracks generally initiate from the insulation layer, it isimperative that the length of the insulation trace being aligned withthe tangent vector of the curvature is minimized In order to prevent orminimize severance of interconnections by cracks in the conductive linetrace 120, the wire trace may split into multiple sub-traces, which andconverge back into a single trace at a certain interval. In the exampleof FIG. 8A, a single trace of a conductive line trace 120 includessub-trace A and sub-trace B, which merge back at every joint X,resembling a chain of diamonds. This trace design may be referredhereinafter as the diamond trace design 800. Because sub-traces arearranged to extend in the vector angled away from the tangent vector ofthe curvature, reduction in the length of the wire trace being alignedwith the tangent vector of the curvature was realized in the similarmanner as the trace designs illustrated in FIG. 7.

The diamond trace design 800 provides significant electrical advantageover the single line wire trace designs of the FIG. 7. First, given thesame width, thickness and the angle shifting away from the tangentvector of the curve, nearly the half of electrical resistance wasobserved from the wire trace employing the diamond trace design incomparison to the wire trace employing the mountain trace design (i.e.,4.4Ω: 8.6Ω). In addition, splitting of the trace into multiplesub-traces may provide a backup electrical pathway in case one of thesub-traces is damaged by cracks. As such, the diamond trace design canbe used for the wire traces in the bend portion, and may be particularlyhelpful for the wire traces within the bend allowance section subjectedto severe bending stress.

As mentioned, the distribution of the bending stress depends on thevector (i.e., split angle) of the sub-traces in reference to the bendingdirection. The sub-trace having a larger split angle away from thebending direction (i.e., tangent vector of the curvature) will besubjected to less bending stress. However, it should be noted that thesplit of the wire trace into multiple sub-traces does not by itselfprovide bend stress reduction on each sub-trace any more than the bendstress reduction realized by the wire trace oriented in the vector angleaway from the tangent vector of the curvature. In fact, given the sameconductive line width and angle of deviation from the tangent vector ofthe curvature, the result of bend stress simulation in a mountain shapedwire trace, which almost mirrors the shape of the one of the sub-tracesin the diamond trace design, was nearly identical to the result of bendstress simulation exhibited on each sub-trace of the diamond tracedesign 800.

However, one of the extra benefits realized from the diamond tracedesign 800 is that the design allows to minimize or to eliminate thelength of insulation layer trace being aligned (i.e., running parallel)to the tangent vector of the curvature with relatively little increasein the electrical resistance. As a result, significantly lower crackinitiation rate can be obtained.

Reduction of the insulation layer trace aligned to the tangent vector ofthe curvature can be done by reducing the width of the conductive linetrace 120 and the insulation layer covering the conductive line trace120. When the insulation layer trace aligned to the tangent vector ofthe curve is eliminated by reduction of conductive line width and theinsulation trace width, the average size of cracks was reduced from 3.79μm to 2.69 μm. The ohmic contact resistance was increased to 4.51Ω from3.15Ω, but such an increase has minimal effect in the operation of theflexible display 100.

The amount of reduction in the width of conductive line trace 120 islimited with the single line trace designs depicted in FIG. 7 as theelectrical resistance of the conductive line trace 120 can become toohigh to be used for the flexible display 100. However, the additionalelectrical pathway created by splitting and merging of the conductiveline trace 120 yields much lower electrical resistance in the conductiveline trace 120 as compared to using the non-split strain reducing tracedesigns.

It should be noted that the splitting angle of the sub-traces affectsthe distance between the two adjacent joints X in the diamond tracedesign 800. The distance between the joints X need not be uniformthroughout the entire wire trace. The intervals at which the tracesplits and merges can vary within a single trace of wire based on thelevel of bending stress exerted on the parts of the wire trace. Thedistance between the joints X may be progressively shortened down forthe parts of the wire trace towards the area of the flexible display 100subjected to higher bending stress (e.g., area having lower radius ofcurvature, area having larger bend angle). Conversely, the distancesbetween the joints X can progressively widen out towards the areasubjected to lower bending stress.

In an exemplary trace design of FIG. 8B, the distance between the jointsX of a trace in the end sections is at a first distance (e.g., 27 um),but the distance becomes progressively shorter towards the mid-sectionof the trace. In the mid-section, the distance between the joints X isreduced by half The end sections of the trace shown in FIG. 8B may befor the part of the wire trace near the beginning of a bend allowancesection, and the mid-section of the trace may be for the part positionedat or near the middle of the bend allowance section of the flexibledisplay 100.

A lower chance of crack initiation is afforded in the wire trace byselectively increasing the angle of sub-traces in the wire trace at highbending stress regions. With sub-traces that split and merge at agreater angle away from the bending direction, more thorough reductionin the lengths of the conductive line trace 120 and the insulation layerextending along the tangent vector of the curvature. This way,unnecessary increase in the electrical resistance can be avoided.

The wire trace may split into additional number of sub-traces, creatinga grid-like wire trace in the bending area of the flexible display 100as illustrated in FIG. 8C. As an example, the sub-traces can beconfigured to form a plurality of a web formed of diamond trace shapes.Such trace design may be useful for wire traces that transmit a commonsignal, for example VSS and VDD. Neither the number of sub-traces northe shape of the sub-traces forming the grid-like trace design areparticularly limited as the example shown in FIG. 8C. In someembodiments, the sub-traces may converge into a single trace past thebend allowance section of the flexible display 100.

The strain reducing trace designs discussed above may be used for all orparts of the conductive line trace 120. In some embodiments, the part ofconductive line trace 120 in the bend portion of the flexible display100 may adopt such a strain reducing trace design. The parts of aconductive line trace 120 prior to or beyond the part with the strainreducing trace design may have the same trace design. If desired, thestrain reducing trace designs may be applied to multiple parts of aconductive line trace 120.

Even with the strain reducing trace design, the inevitable bendingstress remains at certain points of the trace (i.e., stress point). Thelocation of stress point is largely dependent on the shape of the traceas well as the bending direction. It follows that, for a given bendingdirection, the trace of a wire and/or an insulation layer can bedesigned such that the remaining bending stress would concentrate at thedesired parts of their trace. Accordingly, a crack resistance area canbe provided in a trace design to reinforce the part of the wire tracewhere the bend stress concentrates.

Referring back to FIG. 8A, when a wire trace having the diamond tracedesign is bent in the bending direction, the bending stress tends tofocus at the angled corners, which are denoted as the stress point A andstress point B. When a crack forms at those angled corners, it generallygrows in the transverse direction that to the bending direction. Forinstance, at the stress points A, a crack may initiate from the outertrace line 820 and grows towards the inner trace line 830. Similarly, acrack may initiate from the outer trace line 830 and grow towards theinner trace line 820 at the stress points B.

Accordingly, the width of the conductive line trace 120 at the stresspoints A can be selectively increase in transversal direction to thebending direction, thereby serving as a crack resistance area. That is,the widths (W_(A), W_(B)) of the conductive line trace 120 at the stresspoints A and B, which are measured in the crack growth direction, may belonger than the width (W) of the conductive line trace 120 at otherparts as depicted in FIG. 8A. The extra width in the crack growthdirection at the stress points makes the conductive line trace 120 tohold out longer before a complete disconnection occurs.

In a testing, the wires had the three-layered structure (MO 200 Å/AL3000 Å/MO 200 Å), which were formed on a 17 um thick PI base layer 106.A 1000 Å thick SiN_(x) layer was formed between the base layer 106 andthe multi-layered conductive line trace 120. Also, a layer of SiO₂ wasformed over the multi-layered conductive line trace 120. The thickestportion of the SiO₂ on the conductive line trace 120 was 3000 Å. Each ofthe conductive lines 1 through 4 had different width a width of 8.5 um,2.5 um, 3.5 um and 4.5 um, respectively, at the stress points A.

For each wire trace, electrical resistance was measured initially uponthe bending and again at 15 hours later. If a crack is generated in theconductive line trace 120, the resistance of the conductive line trace120 will be increased as well. The wire trace 1 with the longest widthat the stress points A exhibited the lowest mean increase resistancerate whereas the wire 2 with the shortest width at the stress points Aexhibited the largest mean increase resistance rate. Also, a completeseverance was observed in three samples of the wire trace 2 and twosamples of the wire trace 3. While complete severance in the wire trace4, a considerable increase in the resistance was observed after 15hours. Accordingly, a sufficient width at the stress points A is neededto maintain the reliability of the wire.

For instance, the width of the wire at the stress points A may be longerthan 4.0 um. The width of the wire measured in the direction of thecrack growth direction may be longer than 5.0 um for further improvementin the reliability. Even with the increased width of the conductive linetrace 120 in the transversal direction to the bending direction, thelength for the continuous portion of the insulation layer being alignedto the bending direction should be kept minimal Accordingly, in anembodiment, the width of the wire at the stress points A ranges fromabout 2.5 um to about 8 um, more preferably, from about 3.5 um to about6 um, more preferably from about 4.5 um to about 8.5 um, and morepreferably at about 4.0 um.

The width of the conductive line trace 120 measured in the crack growthdirection at the stress points B should also be maintained in thesimilar manner as the width of the conductive line trace 120 at thestress points A. As such, the width of the wire at the stress points Bmay ranges from about 2.5 um to about 8 um, more preferably, from about3.5 um to about 6 um, more preferably from about 4.5 um to about 8.5 um,and more preferably at about 4.0 um. Due to the close proximity of theangled corners and their crack growth direction at the stress points B,the width of the conductive line trace 120 at the stress points B may belonger than width at the stress points A.

In order to minimize the chance of crack initiating from both the innertrace line 820 and the outer trace line 830, at least one of the tracelines be not as sharply angled as the other trace lines at the stresspoints A. In the embodiment depicted in FIG. 8A, the inner trace line820 at the stress points A has the angled corner and the outer traceline 830 at the stress points A is substantially parallel (e.g., ±5°) tothe bending direction. However, the length L of the outer trace line 830extending in the bending direction in excess may defeat the purpose ofutilizing the strain reducing trace design in the first place. As such,the length L for the portion of the outer trace line 830 extendingsubstantially parallel to the bending direction may be equal to ordeviate slightly (e.g., within ±2.5 μm) from the width W of the wiretrace. Alternatively, the sharply angled corner can be formed with theouter trace line 830 while the inner trace line 820 at the stress pointsA being substantially parallel to the bending direction. In both cases,the less sharply angled trace line can simply be more rounded ratherthan having the straight line trace as shown in FIG. 8A.

As discussed above, splitting and merging of the wire creates stresspoints that share the given amount of bending stress. With therelatively low bending stress at each stress point, there is less chanceof crack initiation. In some cases, however, available space on theflexible display 100 may limit the number of joints X of a trace. Thatis, excess joints X in a wire trace may take up too much space in theflexible display 100. On the other hand, the limited number of joints Xin a trace may not be enough to prevent or minimize crack initiating atthe stress points.

Accordingly, in some embodiments, a trace may be provided with a numberof micro-stress points 810 that are strategically positioned along oneor more sub-traces such that the bending stress on the sub-trace isdistributed among the micro-stress points 810. In the example depictedin FIG. 8D, the insulation trace includes a number of micro-stresspoints 810. As discussed, the angled corners tend to be the stresspoints in a trace design. Thus a plurality of angled cutouts can beformed along the insulation layer trace to serve as a micro stresspoints 810. In this setting, at least some fraction of the bendingstress on each of the sub-traces would be focused on each of themicro-stress points 810. With each micro-stress points 810 taking up thefraction of the given bending stress on the sub-traces, the size of thecrack at each micro-stress points 810 may be smaller than a crack sizethat would result in the insulation layer trace without the micro-stresspoints 810. Accordingly, this can reduce the chance of completeseverance of the conductive line trace 120.

It should be appreciated that the location and the number ofmicro-stress points 810 is not limited as shown in FIG. 8D. Additionalmicro-stress points 810 can be formed at the desired location in therespective insulation traces to further reduce the chance of crackinitiation.

As discussed above, some structural elements may not exist in some areasof the flexible display 100 to facilitate bending. For example, elementssuch as the touch sensor layer 112, polarizer layer 110 and the like maybe absent in the bend area of the flexible display 100. Also, some ofthe insulation layers, for instance a multi-buffer layer 118, may besimplified in some degree so that the insulation layer has less numberof sub-layers or has a decreased thickness at one area as compared toother areas in the flexible display 100. Absence or simplification ofthese components and the layers would create a recessed area where thewire trace and/or the insulation layer trace need to cross.

The amount of bending stress and the direction in which the bendingstress is exerted on the wire trace laid over the recessed area maydiffer from the bending stress exerted to other parts of bend portion.To accommodate the difference, the strain reducing trace design for thewire traces at the recessed area can also differ from the strainreducing trace design used elsewhere.

FIG. 9A illustrates a cross-sectional view at an edge of a backplane forin the exemplary flexible display 100, in which several insulationlayers are removed from the bending portion to facilitate more reliablebending.

As shown, there are several organic and inorganic layers formed inbetween the base layer 106 and the OLED element layer 102. In thisparticular example, alternating stacks of SiN_(x) and SiO₂ layers can bedisposed on the base layer 106 to serve as a buffer layer. Thesemiconductor layer of a TFT may be sandwiched by an active-buffer layerand a gate insulation layer that are formed of SiO₂ layer. The gate ofthe TFT is disposed on an interlayer dielectric layer (ILD), and thesource/drain of the TFT having the multi-layered structure as discussedabove is sandwiched between the ILD and a passivation layer. Here, theILD may be formed of a stack of SiN_(x) and SiO₂, and the passivationlayer is formed of SiN_(x). Then, a planarization layer is disposed overthe passivation layer so that the anode for the OLED can be disposedthereon.

It should be noted that the use of the strain reducing trace design isnot just limited to the part of the wires within the bend portion. Inother words, the strain reducing trace design can start and end in thearea outside the bend portion. Using the strain reducing trace designfor the wire trace in such abridging area can afford increasedprotection to the wire trace against the bending stress.

In the abridging area, however, several layers between the base layer106 and the OLED element layer 102 are absent to facilitate bending ofthe flexible display 100. For instance, the ILD and the gate insulationlayer is etched away in the trimmed area by the first etch process,which is followed by the second etch process that etches away the activebuffer and a part of the buffer 126 (e.g., a stack of a SiN_(x) layerand a SiO₂ layer). These etching processes create multiple steps where asharp change of direction occurs between the wire trace disposed on thevertically sloped surfaces and the wire trace disposed on thehorizontally leveled surfaces. In other words, the wire trace would haveseveral bent spots, such as EB1 and EB2.

When bending the flexible display 100 in the bending direction, the wiretrace may experience more strain at or near the steps. Numerous testsand experiments indicate that the chance of a crack is especially highin the wire trace crossing over the step between the EB1 area and theEB2 area. Accordingly, in some embodiments, the strain reduction tracedesign for the wire trace has a reinforced portion at or near the stepbetween a high leveled surface and a low leveled surface provided byinsulation layers of the flexible display.

In the example shown in FIG. 9B, the wire trace has a simple straightline trace design in the beginning, which is changed into the split andmerge strain reduction trace design in the abridged area. In addition,the part of the conductive line that crosses over before and after thebent spots EB1 and EB2 is reinforced with extra width W_(R). That is,the conductive line has substantially wider width to reinforce theconductive line trace 120 near the bent spots EB1 and EB2 to ensure theperseveration of the conductive line trace 120 even if cracks initiatefrom the insulation layer covering the reinforced portion of theconductive line. The distance D_(R) of the reinforced portion of whichthe conductive line is reinforced with the wider increased width W_(R)depends on the size of the steps created by the etching processes aswell as the distance between the bent spots EB1 and EB2. Past thereinforced part, the wire trace continues with the diamond trace designdiscussed above. The strain reduction trace design for the wire tracethat comes before and after the reinforced portion is not particularlylimited to the trace design as depicted in FIG. 9B, and any other strainreduction trace design discussed above may be used.

While this may not always be the case, the abridged area would likely belocated slightly towards the central portion of the flexible display 100before the bend allowance section as it will help the most in bending ofthe flexible display 100. In such cases, the bent spots EB1 and EB2would be positioned at of just outside start of the bend allowancesection in the bend portion.

The increased width W_(R) of the reinforced conductive line trace 120portion may serve its purpose well at or near the beginning of the bendallowance section where the curvature is relatively small. However, thewider width W_(R) of the wire trace would increase the length of thewire trace that is linear to the bending direction. This would be makethe wire trace harder to hold out against the bending stress at theregion with greater bend radius. For this reason, the distance D_(R) inwhich the reinforced portion is used should be limited such that thereinforced conductive line portion does not extend too far beyondtowards into the bend allowance section. In other words, the distanceD_(R) of the reinforced conductive line portion may be limited such thatthe trace design of the reinforced conductive line portion does notextend beyond the bend allowance section with more than a threshold bendangle. In way of an example, the reinforced conductive line portion maynot extend beyond the point where it is 30° curved away from the tangentplane of the curvature. The threshold bend angle may be less than 20°,for example 10°, and more preferably less than 7°.

The wire trace having the reinforced section may extend beyond the bendallowance area and reach into the secondary active area. In suchinstances, there may be additional bent spots (similar to EB1 and EB2)at or near the end of the bend allowance section. The conductive line ator near such bent spots may be reinforced in the similar manner as thewire trace portion at the bent spots EB1 and EB2. If desired, thereinforced conductive line portion at or near the bent spots at theother end of the bend allowance section may be different as depicted inFIG. 9B.

Although the concepts and teachings in the present disclosure aredescribed above with reference to OLED display technology, it should beunderstood that several features may be extensible to any form offlexible display technology, such as electrophoretic, liquid crystal,electrochromic, displays comprising discreet inorganic LED emitters onflexible substrates, electrofluidic, and electrokinetic displays, aswell as any other suitable form of display technology.

As described above, a flexible display 100 may include a plurality ofinnovations configured to allow bending of a portion or portions toreduce apparent border size and/or utilize the side surface of anassembled flexible display 100. In some embodiments, bending may beperformed only in the bend portion and/or the bend allowance sectionhaving only the conductive line trace 120 rather than active displaycomponents or peripheral circuits. In some embodiments, the base layer106 and/or other layers and substrates to be bent may be heated topromote bending without breakage, then cooled after the bending. In someembodiments, metals such as stainless steel with a passive dielectriclayer may be used as the base layer 106 rather than the polymermaterials discussed above. Optical markers may be used in severalidentification and aligning process steps to ensure appropriate bendsabsent breakage of sensitive components. Components of the flexibledisplay 100 may be actively monitored during device assembly and bendingoperations to monitor damage to components and interconnections.

Constituent materials of conductive line trace 120 and/or insulationlayers may be optimized to promote stretching and/or compressing ratherthan breaking within a bending area. Thickness of a conductive linetrace 120 may be varied across a bending area and/or the bend allowancesection to minimize stresses about the bend portion or the bendallowance section of the flexible display 100. Trace design ofconductive line trace 120 and insulation layers may be angled away fromthe bending direction (i.e., tangent vector of the curvature),meandering, waving, or otherwise arranged to reduce possibility ofseverance during bending. The thickness of the conductive line trace120, insulation layers and other components may be altered or optimizedin the bend portion of the flexible display 100 to reduce breakageduring bending. Bend stresses may be reduced by adding protectivemicro-coating layer(s) over components in addition to disclosedencapsulation layers. Conductive films may be applied to the conductiveline trace 120 before, during, or after bending in a repair process.Furthermore, the constituent material and/or the structure forconductive line trace 120 in a substantially flat area of a flexibledisplay 100 may differ from the conductive line trace 120 in a bendportion and/or the bend allowance section.

These various aspects, embodiments, implementations or features of thedescribed embodiments can be used separately or in any combination. Theforegoing is merely illustrative of the principles of this invention andvarious modifications can be made by those skilled in the art withoutdeparting from the scope of the invention.

1. An flexible display, comprising: a substantially flat portion havinga thin-film transistor (TFT) disposed on a base layer and a buffer layerinterposed between the base layer and a semiconductor layer of the TFT;a bend portion having a protruded area and a recessed area, wherein theprotruded area includes the buffer layer disposed on the base layer andthe recessed areas has the base layer without the buffer layer disposedthereon; and a conductive line trace having a portion laid in thesubstantially flat portion of the flexible display and another portionlaid in the bend portion of the flexible display.
 2. The flexibledisplay of claim 1, wherein the buffer layer is formed of one or morelayers of an inorganic material.
 3. The flexible display of claim 2,wherein the buffer layer in the substantially flat portion of theflexible display is thicker than the buffer layer disposed in the bendportion of the flexible display.
 4. The flexible display of claim 3,wherein the buffer layer disposed in the substantially flat portion ofthe flexible display includes at least one additional sub-layer than thebuffer layer disposed in the bend portion of the flexible display. 5.The flexible display of claim 4, wherein the thickness of the bufferlayer in the bend portion of the flexible display ranges from about 100Å to about 2000 Å.
 6. The flexible display of claim 5, wherein the baselayer at the recessed area has a lower thickness than that of the baselayer at the protruded area.
 7. The flexible display of claim 6, whereinthe conductive line trace in the bend portion includes a plurality ofsub-traces that split and merge at predetermined angles away from atangent vector of the curvature of the bend portion.
 8. The flexibledisplay of claim 7, wherein the conductive line trace laid in the bendportion of the flexible display is positioned on the protruded area. 9.The flexible display of claim 8, further comprising a passivation layerthat covers the conductive line trace in the substantially flat portionand the bend portion of the flexible display with a margin of about 2 umextending out from an outline of the conductive trace line trace. 10.The flexible display of claim 9, wherein the recessed area in the bendportion is substantially free of the passivation layer.
 11. The flexibledisplay of claim 7, wherein the conductive line trace laid in the bendportion of the flexible display includes a part disposed on the baselayer of the recessed area and another part disposed on a portion of thebuffer layer of the protruded area surrounding the recessed area. 12.The flexible display of claim 11, further comprising a passivation layerthat covers the conductive line trace disposed on the portion of thebuffer layer of the protruded area without covering the conductive linetrace disposed on the base layer of the recessed area.
 13. A flexibleorganic light emitting diode (OLED) display, comprising: a substantiallyflat portion having a base layer, a buffer layer on the base layer and athin-film transistor (TFT) disposed on the buffer layer; a bend portionhaving the base layer and the buffer layer on the base layer, the bendportion being curved away from a tangent plane of the substantially flatportion by a bend angle; and a conductive line trace extending from thesubstantially flat portion to the bend portion of the flexible display,the conductive line trace in the bend portion having a bend stressrelief trace design in which the conductive line trace splits into aplurality of sub-traces that merge back at one or more intervals,wherein the buffer layer in the bend portion corresponds to the bendstress relief trace design of the conductive line trace in the bendportion.
 14. The flexible OLED display of claim 13, further comprising apassivation layer covering the conductive line trace, wherein thepassivation layer in the bend portion corresponds to the bend stressrelief trace design of the conductive line trace in the bend portion.15. The flexible OLED display of claim 14, wherein at least some of thebase layer in the bend portion of the flexible OLED display is exposedwithout the buffer layer and the passivation layer disposed thereon. 16.The flexible OLED display of claim 15, further comprising amicro-coating layer in the bend portion of the flexible display, themicro-coating layer covering the exposed base layer and the passivationlayer on the buffer layer.
 17. A method of manufacturing a flexibledisplay, comprising: forming a buffer layer on a base layer; forming asemiconductor layer of a thin-film-transistor (TFT) on the buffer layer;forming a gate insulation layer over the semiconductor layer; forming agate electrode layer on the gate insulation layer; forming an interlayerdielectric layer on the gate electrode layer; etching one or more of theinterlayer dielectric layer, the gate insulation layer and a part of thebuffer layer from a bend portion of the flexible display; forming aconductive line trace such that a part of the conductive line trace tobe laid in the bend portion of the flexible display has a bend stressrelief trace design in which the conductive line trace splits into aplurality of sub-traces that merge back at one or more intervals;forming a passivation layer over the bend portion of the flexibledisplay; etching the passivation layer and the buffer layer around theconductive line trace such that the base layer is exposed around theconductive line trace; and forming a micro-coating layer on the exposedbase layer and on the passivation layer on the conductive line trace.